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May 11th 05, 08:13 AM
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May 12th 05, 06:23 AM
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From: Paul Keinanen on Wed,May 11 2005 12:13 am
On 10 May 2005 13:59:13 -0700, wrote: From: Paul Keinanen on 10 May 2005 09:11:19 -0700 In a radio receivers, the signal levels vary from less than a microvolt to several volts, so the crosstalk issues are much more demanding. I will disagree on radio receivers on such wide dynamic ranges. "Several volts" INTO a receiver front end? No. Such levels aren't encountered in practical locations and would, definitely, cause enough IM that would create much distortion and spur products. Look at a multitransmitter contest site with one transmitter on each band, the voltage induced to the receiving antennas for other bands can be quite large. In a production model receiver? Mais non. That's not a design prerequisite, never was, not even with the Rhode & Schwarz designs featuring very high 3rd IP specifications. I've been IN such situations on aircraft installations where the potential RFI was much stronger than in ham DXpedition or Field Day setups. The work-arounds to make the receivers operate is NOT a design criteria, not in avionics-oriented design plans. Of course, in a competent receiver design only the frequency band of interest is filtered out before processing. However, if the antenna is connected directly to the backplane and the modules do their own filtering, the large composite signal on the backplane will radiate all around the system. Possibly, IF and only IF the antenna IS connected to the "backplane" (or motherboard). Why must it be so? Look at the PC. Sound cards have their audio input (at microphone levels) on a separate connection). No interference doing that. In non-contest sites large wire or log-periodic antennas can collect a quite large signal voltage (in the order of 0 dBm, 220 mV or more). Perhaps, but that still isn't a design criterion for present-day ham receivers. Also if the final IF is within or below the receiver tuning range and a diode ring mixer is used as the SSB demodulator with +7 or +17 dBm, you must keep this BFO signal and harmonics from entering the front end. Yes...but that was a problem a half century ago, too! :-) Even the SDR is going to need some switchable front end band pass filters in order to survive in the hostile RF environment these days with a lot of strong signals even in ordinary sites. Diode switching. My two-decade old Icom R-70 has that to select approximate octave-bandwidth bandpass filters to cover 50 KHz to 30 MHz. Has its own little PCB, probably because every single L, C, diode, and resistor is included on that board...no shielding except from the side wall of the cabinet and part of the cast frame. I've had that little receiver within a city block from AM BC station KMPC running 50 KW into its towers. Worked fine with a temporary long-wire antenna despite the RF around that station. In transceivers, there would be several points that would need switching. Of course. That's what was done two decades ago. I used the CANbus as an example, since the cable can be tens or hundreds of meters long depending on speed and thus, it could be used to control some internal points in a transceiver as well as wire all devices in the ham shack as well as in the tower. For instance, the same controller could control the antenna rotator, command the antenna preamplifier to bypass mode, turn the transvertter into transmit mode, select the VFO frequency for transmit (in split operation) and finally turn the transmitter on. Right, no problem...except for the individual ham installer who then has to set up the "program" to do all those things. Can they? :-) I think a better approach is something like SGC does in their automatic antenna tuners. They add a frequency meter function to their tuner micro- controller, a small section of Flash memory to hold data, measure an RF input, adjust the coupler switches to compensate for VSWR, then record that data in memory. Any future frequency close to the recorded memory can use the same settings. Near-ultimate in modularity is thus achieved. Needs only DC power to operate and doesn't care what kind of transmitter is connected to it...as long as its in specification for power and frequency. Absolutely "plug-and-play!" :-) What is lacking is STANDARDIZATION. This is definitely a big problem. Yes and no. :-) It's like a recipe for "tiger soup:" "First, you have to catch a tiger..." In a similar way, there must be SOME idea of what kind of control range, modulation, etc., etc. would be expected...and for what radio service. The FCC in the USA can't yet come to grips on that, nor has industry made much progress outside of their own product lines. Right now, it is more like Pandora's Box. That can't be worked out in newsgroups, A newsgroup is a good place for open ended discussions between people with experience in quite different fields. I agree. But, like the infamous "John Smith," it can be infiltrated with someone who doesn't have either the experience or the courage to use his/her real name. Raises the noise level enough to make some go QRT for a while. Writing a formal specification may require some formal organisation, but on the other hand quite a few successful RFCs in the IT sector are written by a single person or a small group. Ahem...that INDUSTRY specification is going to range considerably farther than some small group within one company. As to IT (Information Technology), I've not seen ANY industry-wide softwares which extend beyond corporate levels and that's been for the last three decades. LANGUAGES not counted there. and willingness to compromise That is the problem in formal committees, in which most delegates from various vendors have large commercial interests in the subject and in order to be able to produce even some kind of standard, all features from various vendors are included. I'll just cite the ARINC standards which are generally used internationally for all civil avionics, from radio to radar, radionavigation systems. ALL the interfaces to every avionics box and the physical shape and mountings. NOT a big commercial venture in terms of profit. If you've been able to read the verbatim minutes of ARINC meetings (I have), then you would see that it can be done. ARINC = Aeronautical Radio INCorporated, once a radio communications provider for airlines, later evolving into a combined industry-government central standards organization for civil avionics. [they have a website, BTW, but the documents are horribly expensive now...] |
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